Plasma display screen comprising a green phosphor

ABSTRACT

The invention relates to a plasma display screen comprising a carrier plate, a transparent front plate, a ribbed structure which divides the space between the carrier plate and the front plate into plasma cells that are filled with a gas, one or more electrode arrays to generate corona discharges in the plasma cells, and a phosphor layer which comprises a phosphor selected from the group composed of doped terbium(III)-activated phosphors of the general formula (In 1-x-y-z Gd y Y z )BO 3 : Tb x , where 0&lt;x≦0.5 and 0&lt;y≦0.5, 0≦z≦0.5 and 1-x-y-z&gt;0. The invention also relates to a phosphor selected from the group composed of doped terbium(III)-activated phosphors of the general formula (In 1-x-y-z Gd y Y z )BO 3 : Tb x , where 0&lt;x≦0.5 and 0&lt;y≦0.5, 0≦z≦0.5 and 1-x-y-z&gt;0.

The invention relates to a plasma display screen comprising a carrierplate, a transparent front plate, a ribbed structure which divides thespace between the carrier plate and the front plate into plasma cellswhich are filled with a gas, one or more electrode arrays to generatecorona discharges in the plasma cells, and a phosphor layer comprising agreen-emitting phosphor of the group composed of doped terbium-activatedborate-containing phosphors.

The principle on which a plasma display screen is based is that a highvoltage in a gas with a low gas pressure causes electromagneticradiation to be generated, which radiation may be visible itself or canbe converted into visible light by phosphors.

In a color display screen of customary design, the gas filling iscomposed of an inert gas, for example xenon, or an inert gas mixturesuch as a mixture of helium, neon and xenon. The discharge processinvolves the generation of ultraviolet radiation in the VUV range, i.e.radiation having a wavelength below 200 nm. This VUV radiation excitesthe red, green and blue-emitting phosphors (RGB phosphors) in thephosphor layer, thereby causing them to emit visible light in the colorsred, green and blue. Thus, unlike conventional fluorescent lamps, theluminescent materials in plasma display screens use the high energy sideof the UV spectrum. Dependent upon the composition of the inert gasmixture and the gas pressure, the VUV emission can vary between a singleline at 147 nm and a broad band in the region of 172 nm. What resultsfrom this is that new demands are imposed on the RGB phosphors in aplasma display screen.

The RGB phosphors constitute the final member of the energy-transferchain wherein electric energy is converted into visible light in theplasma display screen. A factor that has a decisive influence on theefficiency of a plasma display screen with a phosphor layer is theelectro-optical efficiency of the phosphors, i.e. how completely the UVlight generated is absorbed in the phosphor and how completely thevisible light generated subsequently leaves the plasma display screen inthe direction of the viewer. The phosphors for red, green and blue,however, have different properties in terms of long-term behavior, butalso their color saturation and perceptibility are different.

CN 1276406 discloses a green phosphor for plasma display screens of thechemical formula (Ln_(1-x-y-z))Tb_(x)R_(y)Li_(z))[(1−M)PO₄ mBO₃], whichphosphor comprises at least a rare earth metal Yb, La, Gd and Lu,phosphates of Li and B, a main activator (Tb ions) and a co-activator(Ce, Pr and Dy ions), and which phosphor is manufactured by calcinationat a high temperature. If said green phosphor is excited by VUV light,short-wave UV light, electron rays and X-rays, it emits strong greenlight with an emission maximum at 544 nm.

A drawback of these phosphors for generating said green color radiationresides in that their color point and color saturation are not optimalfor plasma display screens.

It is an object of the invention to provide a plasma display screencomprising a carrier plate, a transparent front plate, a ribbedstructure dividing the space between the carrier plate and the frontplate into plasma cells which are filled with a gas, one or moreelectrode arrays for generating corona discharges in the plasma cells,and a phosphor layer comprising a green-emitting phosphor of the groupcomposed of doped terbium-activated phosphors, which plasma displayscreen is characterized by an improved, faithful color rendition and ahigher brightness.

In accordance with the invention, this object is achieved by a plasmadisplay screen comprising a carrier plate, a transparent front plate, aribbed structure dividing the space between the carrier plate and thefront plate into plasma cells which are filled with a gas, one or moreelectrode arrays for generating corona discharges in the plasma cells,and a phosphor layer comprising a phosphor selected from the groupcomposed of doped terbium(III)-activated phosphors of the generalformula (In_(1-x-y-z)Gd_(y)Y_(z))BO₃:Tb_(x), where 0<x≦0.5 and 0<y≦0.5,0≦z≦0.5 and 1-x-y-z>0.

In such a color display screen, the green point has been shifted toattain a higher color saturation. This has an influence not only on thegreen tones but also on all intermediate tones on the blue-green andred-green lines which become available as a result of the enlargement ofthe display triangle in the green range. By virtue thereof, a morefaithful display of many color tones is possible, resulting in a visibledifference. In addition, the color contrast under bright ambient lightconditions is increased.

The invention also relates to a phosphor selected from the group ofdoped terbium(III)-activated phosphors of the general formula(In_(1-x-y-z)Gd_(y)Y_(z))BO₃:Tb_(x), where 0<x≦0.5 and 0<y≦0.5, 0≦z≦0.5and 1-x-y-z>0.

This phosphor is characterized by improved thermal loadability,particularly in an oxygen-containing atmosphere, which can be attributedto the fact that Tb(III) does not have a tendency to oxidize to Tb(IV).As a result, the luminance of this phosphor is not adversely affectedduring the manufacture of the plasma display screen, and the efficiencyremains constant for a very long time even in the case of excitation byVUV radiation.

These and other aspects of the invention are apparent from and will beelucidated with reference to a drawing and two examples.

In the drawing:

FIG. 1 shows a plasma color display screen of the surface discharge-typewhich is composed of a system of layers which are arranged above oneanother and partly next to each other.

In a plasma display screen of the surface discharge type, light isgenerated in a plasma by a gas discharge in a three-electrode system.Said three-electrode system comprises an address electrode and twodischarge electrodes per pixel, between which an alternating voltage isapplied during operation.

Such a plasma color display screen of the surface discharge type iscomposed of a transparent front plate 1 and a carrier plate 2, which arearranged at a distance from each other and are hermetically sealed atthe periphery. The space between the two plates constitutes thedischarge space 3 which is bounded by the protective layer and thephosphor layer. Customarily, the front plate as well as the carrierplate are made of glass. Individually drivable plasma cells are formedby a ribbed structure 13 having separating ribs. A plurality oftransparent picture electrodes 6, 7 are arranged in the form of stripeson the front plate. The associated control electrodes 11 are provided atright angles thereto on the carrier plate, so that a discharge can beignited at each one of the crossing points.

The discharge space is filled with an appropriate discharge gas, forexample xenon, a xenon-containing gas, neon or a neon-containing gas.The gas discharge is ignited between the picture electrodes 6, 7 on thefront plate. To preclude direct contact between the plasma and thepicture electrodes 6, 7, the latter are covered with a dielectric layer4 and a protective layer 5. In the discharge space, the gas is ionizedand a plasma develops which emits VUV radiation.

Dependent upon the composition of the gas in the plasma cell, thespectral intensity of the gas discharge changes. Gas mixtures containingless than 30% by volume xenon emit substantially resonant radiation at147 nm, gas mixtures containing more than 30% by volume xenon emitexcimer radiation at 172 nm.

The emitted VUV radiation excites the structured red, green and bluephosphors pixel by pixel, thereby causing them to emit light in thevisible range, as a result of which a color impression is formed. Thepixels of the plasma display screen in the three primary colors red,blue and green are formed by a phosphor layer 10 on at least a part ofthe carrier plate and/or on the walls of the separating ribs in theplasma cells. The plasma cells are each successively coated with a red,green or blue phosphor. Three juxtaposed plasma cells represent onepixel, which enables all colors to be displayed by mixing said threeprimary colors.

Individually controllable discharge cells are formed by a ribbedstructure with separating ribs. A ribbed structure comprising straight,parallel separating ribs divides the discharge space into uninterruptedvertical strips. A ribbed structure with buckled or corrugatedseparating ribs divides the discharge space into discontinuouschain-type vertically lined up discharge cells of, for example,hexagonal or ellipsoidal cross-section.

Between the separating ribs, the front plate is coated with a phosphorlayer of phosphor segments. A picture element, i.e. a pixel, is definedby the combination of at least three sub-pixels in the colors red, greenand blue. The sub-pixels are formed by the three luminescent phosphorsegments 4G, 4R and 4B in the colors red, green and blue. Threedischarge cells each comprising a red, green and blue phosphor segmenteach form a sub-pixel, and, as a triplet, a picture element.

The pattern of the phosphor segments is determined by the course of theseparating ribs and vice versa. In the embodiment shown in FIG. 1, thephosphor segments form an in-line strip pattern, in which the phosphorsegments form uninterrupted elongated strips. Along a strip, the colorof the phosphor remains unchanged.

In accordance with another embodiment of the invention, the individualphosphor strips may be divided into rectangular phosphor segments(Mondrian pixels) for the three primary colors which are arranged inaccordance with a zigzag pattern or a dovetail pattern.

The phosphor segments for the primary colors red, green and blue eachcomprise a red, green or blue-emitting phosphor. Particular suitablephosphors are phosphors that can be excited by the UV component of theradiation from the gas plasma.

For the red-emitting phosphors that can be excited by VUV radiation usecan suitably be made of (Y,Gd)BO₃:Eu, Y₂O₂S:Eu, Y₂O₃:Eu, Y(VP)O₄:Eu,Y(V,P,B)O₄:Eu, YVO₄:Eu, SrTiO₃:Pr, GdMgB₅O₁₀:Ce,Mn and Mg₄GeO_(5.5)F:Mn.

For the blue-emitting phosphors that can be excited by VUV radiation,use can suitably be made of (Sr,Mg)₂P₂O₇:Eu, Ba,Sr)₅(PO₄)₃Cl:Eu, CaWO₄,(Y,Gd)(P,V)O₄ and Y₂SiO₅:Ce.

For the green-emitting phosphors that can be excited by UV radiation useis made of doped terbium(III)-activated phosphors of the general formula(In_(1-x-y-z)Gd_(y)Y_(z))BO₃:Tb_(x), where 0<x≦0.5 and 0<y≦0.5, 0≦z≦0.5and 1-x-y-z>0.

For the invention use can particularly suitably be made of a combinationof europium-activated barium magnesium aluminate BaMgAl₁₀O₁₇:Eu as theblue-emitting phosphor, europium-activated yttrium oxide Y₂O₃:Eu oreuropium-activated yttrium gadolinium borate (Y,Gd)BO₃:Eu as thered-emitting phosphor and (In_(0.45)Gd_(0.45))BO₃:Tb_(0.1, (In)_(0.2)Gd_(0.7))BO₃:Tb_(0.1) or (In_(0.7)Gd_(0.2))BO₃:Tb_(0.1) as thegreen-emitting phosphor.

The doped terbium(III)-activated phosphors for generating the greencolor radiation having the general formula(In_(1-x-y-z)Gd_(y)Y_(y))BO₃:Tb_(x), where 0<x≦0.5 and 0<y≦0.5, 0≦z≦0.5and 1-x-y-z>0 may have a host lattice having a vaterite structure whichis doped with the activator ion Tb³⁺ in a small concentration. Thevaterite structure has two crystallographically independent latticesites which are occupied by the trivalent cations. One lattice site hasan inversion center, the other does not. The Tb³⁺ ion occupies bothlattice sites.

The doped terbium(III)-activated phosphors for generating the greencolor radiation having the general formula(In_(1-x-y-z)Gd_(y)Y_(z))BO₃:Tb_(x), where 0<x≦0.5 and 0<y≦0.5, 0≦z≦0.5and 1-x-y-z>0 may alternatively have a host lattice having a calcitestructure which is doped with the activator ion Tb³⁺ in a smallconcentration. The calcite structure has only one crystallographiclattice site, which is occupied by the trivalent cations. This latticesite has an inversion center. The Tb³⁺ ion occupies this lattice site.

In compounds having a vaterite structure or a calcite structure, theactivator ion Tb³⁺ has an emission spectrum with three emission lines at490 nm (⁵D₄-⁷F₆), 545 nm (⁵D₄-⁷F₅) and 620 nm (⁵D₄-⁷F₃). In the case ofcustomary phosphors having terbium ions at lattice sites withoutinversion symmetry, saturation of this green light emission is reducedby transitions ⁵D₄-⁷F_(6,4,2).

By virtue of the fact that the host lattice of the phosphors inaccordance with the invention contains the Tb(III)-ions at lattice siteswith inversion symmetry or a small deviation from inversion symmetry,the ⁵D₄-⁷F_(6,4,2) transitions are suppressed and saturation of thegreen light emission is increased without the quantum efficiency beingreduced.

Consequently, the green emission multiplet at 545 nm is very intensiveand exhibits a high lumen equivalent >500 lm/W.

The terbium(III)-activated rare earth metal borates are manufactured bymeans of conventional methods, for example by means of a solid statereaction. In the manufacturing process, the oxides or carbonates areused as the starting compound. They are mixed, ground and subsequentlysintered. As a result, phosphors having a homogeneous crystal structureare obtained in the form of fine-grain particles having a grain sizeranging from 1 to 10 μm.

To manufacture the phosphor layer use can be made of dry coatingmethods, for example electrostatic deposition or electrostaticallyassisted dusting, as well as wet coating methods, for example screenprinting, dispenser methods, wherein a suspension is introduced using anozzle moving along the channels, or sedimentation from the liquidphase.

For the wet coating methods, the phosphors must be dispersed in water,an organic solvent, if necessary in combination with a dispersing agent,a tenside and an antifoaming agent or a binder preparation. Organic andinorganic binders capable of withstanding an operating temperature of250° C. without being subject to decomposition, embrittlement ordiscoloration can suitably be used as the binder preparations for plasmadisplay screens.

Although the invention has been described with reference to a colorplasma display screen of the surface discharge type, the application ofthe invention is not limited to this type of plasma display screen, butalso includes, for example, DC color plasma display screens andmonochromatic AC and DC plasma display screens.

EXAMPLE 1

To manufacture (In_(0.7)Gd_(0.2))BO₃:Tb_(0.1), a quantity of 10.0 g(36.02 mmol) In₂O₃, 3.730 g (10.2 mmol) Gd₂O₃, 1.924 g (2.57 mmol) Tb₄O₇and 7.0 g (113.20 mmol) H₃BO₃ are thoroughly ground in an agate mortar.After a first tempering operation at 700° C., the powder is ground againand sintered twice at 1100° C. in a CO atmosphere. After the firstsintering operation, the powder is ground again. Subsequently, thephosphor is washed with demineralized water, filtered-off and dried at100° C. As a result, a white powder is obtained which is passed througha sieve with a mesh size of 30 μm. The phosphor thus manufactured wasused to manufacture and test a plasma display screen in accordance withthe known methods. The measuring values as listed in Table 1 were found.TABLE 1 Color location x, y LO [147 nm] LO [172 nm] 0.338, 0.615 0.770.75

The phosphor thus manufactured was used to manufacture and test a plasmadisplay screen in accordance with the known methods.

EXAMPLE 2

To manufacture (In_(0.2)Gd_(0.7))BO₃:Tb_(0.1), a quantity of 2.000 g(7.2 mmol) In₂O₃, 10.445 g (28.82 mmol) Gd₂O₃, 2.394 g (3.20 mmol) Tb₄O₇and 8.710 g (140.87 mmol) H₃BO₃ are thoroughly ground in an agatemortar. After a first tempering operation at 700° C., the powder isground again and sintered twice at 1100° C. in a CO atmosphere. Afterthe first sintering operation, the powder is ground again. Subsequently,the phosphor is washed with demineralized water, filtered-off and driedat 100° C. As a result, a white powder is obtained which is passedthrough a sieve having a mesh size of 30 μm. The phosphor thusmanufactured was used to manufacture and test a plasma display screen inaccordance with the known methods. The measuring values as listed inTable 1 were found. TABLE 1 Color location x, y LO [147 nm] LO [172 nm]0.338, 0.615 0.77 0.75

The phosphor thus manufactured was used to manufacture and test a plasmadisplay screen in accordance with the known methods.

EXAMPLE 3

To manufacture (In_(0.45)Gd_(0.45))BO₃:Tb_(0.1), a quantity of 8.0 g(28.82 mmol) In₂O₃, 10.445 g (28.82 mmol) Gd₂O₃, 1.924 g (2.57 mmol)Tb₄O₇ and 7.0 g (113.20 mmol) H₃BO₃ are thoroughly ground in an agatemortar. After a first tempering operation at 700° C., the powder isground again and sintered twice at 1100° C. in a CO atmosphere. Afterthe first sintering operation, the powder is ground again. Subsequently,the phosphor is washed with demineralized water, filtered-off and driedat 100° C. As a result, a white powder is obtained which is passedthrough a sieve having a mesh size of 30 μm. The phosphor thusmanufactured was used to manufacture and test a plasma display screen inaccordance with the known methods. The measuring values as listed inTable 1 were found. TABLE 1 Color location x, y LO [147 nm] LO [172 nm]0.338, 0.615 0.77 0.75

The phosphor thus manufactured was used to manufacture and test a plasmadisplay screen in accordance with the known methods.

1. A plasma display screen comprising a carrier plate, a transparentfront plate, a ribbed structure which divides the space between thecarrier plate and the front plate into plasma cells which are filledwith a gas, one or more electrode arrays for generating coronadischarges in the plasma cells, and a phosphor layer comprising aphosphor selected from the group composed of dopedterbium(III)-activated phosphors of the general formula(In_(1-x-y-z)Gd_(y)Y_(z))BO₃:Tb where 0<x≦0.5 and 0<y≦0.5, 0≦z≦0.5 and1-x-y-z>0.
 2. A phosphor selected from the group composed of dopedterbium(III)-activated phosphors of the general formula(In_(1-x-y-z)Gd_(y)Y_(z))BO₃:Tb_(x), where 0<x≦0.5 and 0<y≦0.5, 0≦z≦0.5and 1-x-y-z>0.